The present invention relates in general to the field of proton exchange membrane fuel cells, and, more particularly, to unitized MEA assemblies and methods for making same.
FIG. 1 is a sectioned representation of a typical PEM fuel cell assembly. Known fuel cell constructions commonly include a proton exchange membrane disposed between respective anode and cathode plates. As shown in FIG. 1, a typical PEM fuel cell comprises an MEA, which itself usually consists of five layers: a PEM membrane 120, two catalytic active layers 116 and 118, and two gas dispersion layers 104 and 108. The anode flow field/separator plate is illustrated at 106; the cathode flow field/separator plate is illustrated at 110. In a typical PEM fuel cell assembly, the flow field/separator plates have manifolding passages for the gases and fluids. As these plates are composed of premium cost materials optimized for their functions, the extension of those plates to incorporate manifolding represents a cost penalty to achieve the desired functionality.
An electrochemical reaction takes place at and between the anode plate and cathode plate, with attendant formation of a product of the reaction between the fuel and oxygen, release of thermal energy, creation of an electrical potential difference between the plates, with the thus generated electric power usually constituting the useful output of the fuel cell. The general principles of operation of such fuel cells are so well known that they need not be discussed here in great detail.
FIG. 2 is another view of the typical components of a conventional PEM fuel cell assembly utilizing conventional serpentine-channeled flow field/separator plates. Flow field/separator plate 202 is located on the anode side of MEA 206; flow field/separator plate 204 is located on the cathode side of MEA 206. Illustrated in both plate 202 and plate 204 is an example of a serpentine channel used to inter alia facilitate reactant dispersion and water management in the fuel cell.
Typically, in a conventional assembly, PEM 208 of MEA 206 is extended beyond the active area 210 of MEA 206 to create a continuous sealing plane between MEA 206 and the adjacent flow field/separator plates 202 and 204. In PEM fuel cells, the PEM of the MEA is usually made from a polymer, such as a perfluorosulfonic acid, which is one of the most expensive components of the MEA. The sealing plane is usually formed so as to reflect the shape of the adjacent flow field/separator plates, and, therefore, may contain manifolds for inter alia anode gas, cathode gas, and heat exchange fluids. For instance, incorporated within sealing plane 208 are manifolding passages 212 that facilitate the flow of reactants and products to and from the individual fuel cells within a stack assembly. As shown in FIG. 2, for example, sealing plane 208 reflects the shapes of flow field/separator plates 202 and 204. Thus, the manifolds for the flow of reactants and products between the flow field/separator plates are not interrupted by the MEA.
Because of the MEA's handling and durability problems, the most critical and difficult sealing interfaces within a cell or a stack are those between the MEA and the adjacent surfaces to which it is sealed. Typically, the MEA has seal paths between itself and the adjacent flow field/separator plates. This seal is generally effected only as a function of complete cell or stack assembly in as much as the sealing is completed by the clamping force exerted on the seals by the end plates, which compress the stack with heavily torqued threaded fasteners. Consequently, the integrity of the seals is only verifiable after assembly is complete. Any compromise in the integrity of the seals requires complete disassembly and subsequent reassembly of the stack. These assembly and reassembly issues pose significant challenges to efficient, higher speed or automated assembly processes.
Due to the thin, delicate, and fragile nature of the PEM, assembling the MEA between the flow field/separator plates of an individual cell and then assembling an entire stack is difficult. Therefore, assembly is usually performed manually.
Nonuniform placement and improper location of the MEA also may lead to a multitude of problems. For instance, misplacement or movement of the MEA within a fuel cell can lead to gas leaks, premature failures, or a reduction in the power output of the MEA, resulting in cost penalties and other quality control pitfalls. If a single cell malfunctions for any of these reasons in a stack assembly, the stack assembly becomes ineffective and usually must be rebuilt. Accordingly, for successful operation of a fuel cell and fuel cell stack assembly, the MEA needs to able to be accurately and repeatedly positioned so that there is a low risk of displacement. Moreover, maintaining the constant placement of the MEA through subsequent assembly operations is key. Heretofore known means and methods of placing the MEA have not been able to attain adequate controlled placement of the MEA.
Accordingly, there is a need for a means to accurately and repeatedly position the MEA within a fuel cell. Among other things, such means should ensure that the MEA stays as placed through all subsequent assembly operations. Such means also should be economical, and preferably, should reduce costs and improve assembly efficiency. Moreover, if such means could allow the assembly of the components of a fuel cell or stack to be automatic rather than manual, this would be desirable.